In firing installations of conventional type of construction, the fuel is injected into a combustion space via a nozzle and burned there with the addition of combustion air. In principle, the operation of such firing installations is possible with a gaseous and/or liquid fuel. When a liquid fuel is used, the weak point with respect to clean combustion in relation to NO.sub.x, CO and UHC emissions (UHC=unsaturated hydrocarbons) primarily lies in the fact that the atomization of the fuel must attain a high degree of mixing (gasification) with the combustion air. When a gaseous fuel is used, the combustion therefore takes place with a substantial reduction in the pollutant emissions. However, in firing installations for heating boilers, gas-operated burners, despite the many advantages, have not really been able to prevail. The reason for this may be that the logistics for gaseous fuels necessitate an infrastructure expensive per se. If the operation of firing installations with liquid fuel is therefore provided, the quality of the combustion with regard to low pollutant emissions is heavily dependent upon whether success is achieved in providing an optimum degree of mixing between fuel and combustion air, i.e. whether complete gasification of the liquid fuel is guaranteed. The use of a premixing section, which acts upstream of the actual burner head, has not achieved the goal, for it must always be feared in the case of such a configuration that a flashback of the flame into the interior of the premixing zone can take place. It is admittedly true that premixing burners have been disclosed which work with 100% excess air, so that the flame can be operated shortly before the point of extinction. Here, however, it has to be taken into consideration that excess air of 15% at most is permissible in firing installations on account of the boiler efficiency, which is why the use of such burners in atmospheric firing installations does not guarantee optimum operation. Furthermore, even if the requisite degree of gasification of the liquid fuel could approximately be achieved, there would still be no effect on the high flame temperatures, which are known to be responsible for the formation of NO.sub.x emissions. The desired combustion at low flame temperatures as well as with a homogeneous fuel/air mixture cannot be achieved with the means which have been disclosed by the prior art.